A prior art plug used for Christmas light bulb series is designed to be connected in series to fit for a multitude of light bulb series using. That is, the plug can be attached to a receptacle and in the same time provides another receptacle on its back to which another plug can be attached so that a plurality of light bulb series can use the same power source, which was described in U.S. Pat. No. 4,846,723 and 4,768,979. However, in the two cases there is no protection mechanism for safety. Accordingly, another improved construction was presented in U.S. Pat. No. 4,904,976, in which a fuse is installed in the plug to prevent an overcurrent in light bulb series and a cover is provided on the housing so that people can open the cover to inspect or replace. But, there still exist shortcomings in such a design. For example, the copper conductor plate is a single whole piece with one projecting end used as a copper blade that is to be inserted into another receptacle, and the other end situated inside the housing and used as a connecting portion that is to reach the copper blades of another plug. The fuse connects the copper conductor plates to the light bulb series and so the current still flows to the next plug even when the fuse is burned down, which is apt to incur danger. Additionally, the cover is not easy to open and if the cover is opened but the plug has not been disconnected due to a user's neglect, the danger of an electric shock may happen.